Graphene is a material with a thickness of a single atom consisting of carbon atoms arranged in a hexagonal lattice shape on a two-dimensional plane. It is a material receiving huge attention from various fields due to its remarkable characteristics such as an excellent electrical mobility, thermal conductivity, mechanical strength, transparency quantized in terms of thickness, and high specific surface area (Park et al., Nature Nanotechnology, 2010, 4, 217-224; Geim, Science, 2009, 324, 1530-1534; Allen et al., Chemical Reviews, 2010, 110, 132-145).
Graphene may be applied to various fields such as automobile, energy, marine, aerospace, construction, electronic products, medicine, military, and communication, as a next generation energy storage material, semiconductor material alternative to silicon, super capacitor, light weight structural parts, electromagnetic shielding material, sensor, display, and the like.
In order to produce commercial-scale graphene in various application fields, a method of producing graphene oxide from graphite and then reducing the graphene oxide back to graphene is currently being investigated. The modified Hummers method is most widely used as the method for producing graphene oxide from graphite.
Methods being used for reducing graphene oxide back to graphene include a method of using a very toxic reducing agent such as hydrazine or dimethyl hydrazine, a heat treatment method of heating a graphene oxide under a reducing atmosphere at 1,000° C. or higher to remove oxygen, a method of partially removing oxygen from the surface of the graphene oxide using relatively weak reducing agents such as ascorbic acid, and a reducing method using an eco-friendly high efficient supercritical alcohols that have started to be researched recently.
Metal oxide nanoparticles are being used in various industrial fields such as electronic materials, catalysts, sensors, inks, and secondary battery electrode materials using their optical, electrical, magnetic, and chemical characteristics that change when the size of the metal oxide is reduced to nano size.
As the method for preparing metal oxide nanoparticles, gas evaporation-condensation methods or methods using ball milling, which have drawbacks of high cost and low efficiency have been mostly used. Furthermore, as the chemical preparing method, a method of preparing metal oxide nanoparticles from a metal precursor in an organic solvent or aqueous solution has been suggested, but this method uses a toxic organic solvent and generates a large amount of waste water after the processing, and thus causes problems in terms of cost, time and environmental pollution.
Meanwhile, in the case of metal oxide nanoparticles, due to the instability of the nanoparticles, the nano size cannot be retained further when used in various chemical reactions or electrode materials, thereby easily changing the natural characteristics of the nanoparticles. In order to improve the performance of such metal oxide nanoparticles, various attempts are being made to produce various types of composites, and recently, there is an ongoing research to produce a metal oxide nanoparticle/graphene composite by depositing metal oxide nanoparticles on a graphene sheet uniformly, so as to significantly improve the low electrical conductivity and ion conductivity that most metal oxides have, and to buffer for the changes in the volume of the metal oxide. Research is being conducted to utilize such a metal oxide nanoparticle/graphene composite in various fields such as an anode of lithium secondary batteries, high performance capacitors, gas sensors, catalysts and the like.
Methods for preparing such a metal oxide nanoparticle/graphene composite currently being used include a method of dispersing graphene oxide in a solvent where a metal oxide precursor has been dissolved, forming metal oxide nanoparticles on the graphene oxide, and then reducing the graphene oxide thermally or by hydrazine; and a method of mixing graphene oxide and metal precursor with a solvent such as DMF (dimethylformamide), NMP (N-methyl-2-pyrrolidone), EG (ethylene glycol), and water, dispersing the mixture, and then putting the mixture in an autoclave to conduct a solvothermal synthesis or hydrothermal synthesis.
However, these methods take a very long reaction time of about 12 to 48 hours, and require an additional calcination process, leading to a problem of high preparation cost. Furthermore, since the process of generating metal oxide nanoparticles and the process of reducing graphene oxide are separately conducted, the whole process becomes a multistep process, which is also a problem for commercial production. Not only that, there is also a problem that the size distribution of the metal oxide growing on the surface of the prepared graphene composite is too broad, and it is difficult to reduce the size to nanometers, thereby deteriorating the overall performance of the composite.
Therefore, it is acutely needed to develop a method for preparing a metal oxide nanoparticle/graphene composite where metal oxide particles are combined on a surface of graphene, the metal oxide particles being dispersed very uniformly in nano sizes by a simplified one step process in a short time using an eco-friendly solvent with a high yield rate.